The present invention relates generally to magnetic resonance imaging and, in particular, relates to a method for positioning a laser beam onto a predetermined location to identify a tissue to be imaged.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated. This signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
In modern MRI systems, a laser beam is used to define the center location, or sweet spot, of an image of human tissue to be imaged. However, due to mechanical interference with the MRI system, the laser emitting diode is typically mounted onto an outer surface of the MRI system. In order to direct the laser beam radially inwardly towards the patient, the beam is deflected off a mirror that is also mounted on the outer surface of the RF coils. An opening extends radially through the outer surface and provides a conduit for the laser beam to deflect off the mirror radially inwardly to identify the portion of the tissue that will correspond to the sweet spot of the image. Accordingly, once the laser has been calibrated, a patient may subsequently be placed in the MRI system and positioned so that the tissue to be imaged is identified by the laser beam. This will ensure that the sweet spot of the image will correspond to the desired tissue.
However, because the diode is positioned at an appreciable distance from the mirror, and because of the relatively small size of the mirror, and due to tolerances associated with manufacturing, it is highly unlikely that the beam will be in initial alignment with the mirror. For example, if the diode is mis-aligned by as little as xc2xcxc2x0, the beam will not hit the mirror. Furthermore, the laser will be periodically re-calibrated due to vibrations associated with operation of the MRI system.
Moreover, the laser must be adjusted to hit the precise point on the mirror that will yield the desired deflection. Accordingly, the position of the diode will need to be adjusted in the x and y directions so that the beam will deflect off the mirror. One method that could be used to properly align the laser with the mirror is to manually translate the diode in the x and y directions. The user will then rely on sight to determine when the beam becomes deflected off the mirror, which will become apparent when the beam extends through the MRI system in a predictable manner, and onto a predetermined calibration location. In particular, an adjustment lever extends from the laser assembly and out of the MRI system housing that may be manipulated to adjust the position of the diode. However, because the laser beam will be hidden when not properly aligned with the mirror, the user will be unaware what positional adjustments to the diode are necessary. Therefore, the user will essentially be blindly moving the diode at random until the beam hits the mirror. This is an unacceptably tedious, cumbersome, and time-consuming process. Furthermore, the sensitivity of the laser assembly hinders the fine adjustment of the laser beam using this method.
What is therefore needed is an improved method and apparatus for reliably and systematically manipulating a laser diode to align the output laser beam with a desired target location.
An apparatus for systematically aiming a laser beam to a target is presented having an outer housing extending generally along a central axis, a laser diode operable to emit the laser beam in the general direction of the target, and an adjustable coupling for mounting the laser diode to the outer housing and being operable to systematically move the laser beam in a search path that intersects with the target.